A brake system of the general type under consideration is known, for example, from DE 103 36 611 A1. In such brake systems, a plurality of pneumatically operated service brake circuits, for example a first service brake circuit for the wheels of the front axle and a second service brake circuit for the wheels of the rear axle, are generally provided. The brake cylinders, which are assigned to the wheel brakes, are activated pneumatically. The pressure necessary for this is made available by a compressed air reservoir vessel assigned to the respective brake circuit. The brake pressure made available in this way is modulated by means of corresponding, electro-pneumatic brake control modules.
In electronic brake systems, these brake control modules receive, inter alia, electrical signals from an electro-pneumatic brake pedal device that has an electrical part and a pneumatic part. The electrical signals are converted into further electrical control signals for controlling electro-magnetic valves by means of which the brake pressure is metered.
A first service brake circuit for the wheels of the front axle that has a corresponding front-axle brake control module is usually provided. In a corresponding way, a second service brake circuit for the wheels of the rear axle of a vehicle with a rear-axle brake control module is provided.
However, the electrical and/or electronic components of such an electronic brake system can fail. Accordingly, such a brake system must be capable of reliably bringing the vehicle to a standstill even when these electrical and/or electronic components fail.
For this reason, conventional brake pedal devices also have pneumatic components in addition to the above-mentioned electrical and/or electronic components. In this way, at the same time, the brake pedal device is used to generate a pneumatic redundancy pressure that can reliably brake the vehicle by means of the pneumatic subsystem when the electrical or electronic part of an electronic brake system fails.
Furthermore, such brake systems have a parking brake device. For this purpose, at least the wheels of an axle, generally of the rear axle, are embodied with combined spring-loaded/diaphragm brake cylinders. If the spring-loaded part is vented, a storage spring engages the brake. It is therefore possible for the vehicle to be securely held even in the pressureless state. In contrast, in the driving mode, the spring-loaded part is ventilated, with the result that the storage spring no longer applies the brake. In this operating state, the vehicle can then be braked by means of the diaphragm part or the service brake part of the combined storage-spring/diaphragm brake cylinder.
The parking brake is therefore conventionally released pneumatically. For this purpose, corresponding pneumatic lines are provided leading from the spring-loaded brake cylinders to electro-pneumatic control units and usually also into the driver's cab, it being possible to ventilate or vent the spring-loaded parts of the combined spring-loaded brake cylinders in the driver's cab by means of corresponding pneumatic valves. In some systems, even though the control of the parking braking function is activated electrically from the driver's cab, the parking brake is still also released pneumatically or applied mechanically by means of a storage spring. It is therefore possible for the piping for the driver's cab for the activation of the parking brake to be reduced. However, at the same time there is still a need for pneumatic piping at least from the electro-pneumatic control unit of the parking brake device to the spring-loaded brake cylinders.
In addition, in known brake systems the service braking function and the parking braking function are largely separated from one another conceptually and structurally so that the two functions are available independently of one another, in particular when one of the two functions fails. Although the driver of the vehicle with such a brake system can brake the vehicle in the event of failure of the service brake device by activating the parking brake device, this is a difficult task for the driver, in particular during critical driving situations, owing to the completely different functioning of the parking brake device. For example, the parking brake device is usually not equipped with anti-lock brake devices so that when the vehicle overbrakes unstable driving states can occur. This is exacerbated, in particular, because the parking brake device is usually activated manually by the driver. However, the braking sensation that the driver experiences with a manual brake differs substantially from the braking sensation occurring with a foot-operated brake. The risk of overbraking or underbraking through manual activation of the parking brake device is therefore particularly great. The two different brake devices of the parking brake and of the service brake in conventional brake systems are therefore only suitable to a limited degree for assuming the function of the respective other brake device in the event of a fault.